Magnetic transducer element and method of preparation

ABSTRACT

A magnetic transducer element is formed in a portion ( 22 ) of a shaft ( 20 ) by an axially-oriented magnetic source ( 30 ) comprising a U-shape permanent magnet or electromagnet assembly, the gap (g) between the poles of which is small compared to the pole width (w). The source ( 30 ) is brought up to the continually rotating shaft ( 20 ) from a distance (D 1 ) to a position closely proximate (D 2 ) the shaft ( 20 ) and then retracting the source ( 30 ). With an electromagnet the mechanical movement may be emulated by controlling the energising of the electromagnet. An annular, surface-adjacent zone of axially-directed magnetisation is created whose external detectable field ( 40 ) has a distribution in the axial direction which shifts axially under applied torque. An axial or radial component may be sensed as a measure of torque. A method of preparing the shaft for magnetisation (magnetic-cleansing) and a post-magnetisation procedure is disclosed.

FIELD OF THE INVENTION

[0001] This invention relates to a torque or force magnetic transducerelement and to a transducer arrangement or system incorporating such anelement. The invention further relates to a method of preparation of amagnetisable member, such as a putative transducer element, formagnetisation and to a method of magnetising a portion of the member soprepared.

[0002] The invention will be particularly discussed and described inrelation to the measurement of torque, and more particularly torqueapplied to a shaft in which the transducer element is an integralportion of the shaft. The shaft is assumed to be of ferromagneticmaterial. The material is preferably chosen to be a hard magneticmaterial capable of achieving a high saturation and remanence and havinga high coercivity.

BACKGROUND OF THE INVENTION

[0003] There have been previous proposals for a transducer element to beintegrally formed in a portion of a ferromagnetic shaft subject totorque about its lengthwise axis. One previous proposal is disclosed inPCT application PCT/GB00/03119 published under the number WO00/13081 isto longitudinally magnetise the shaft portion so as to create a torus ofaxially-directed magnetisation. Torque is sensed by detecting atangentially or circumferentially-directed component of magnetic field,which component is torque dependent. In order to assist an understandingof the present invention, there will first be outlined an implementationof the previous proposal with reference to FIGS. 1-3 c of theaccompanying drawings.

[0004]FIG. 1 shows how an annulus of longitudinal magnetisation isapplied to an integral portion of a shaft. The portion is to provide atransducer element and it at least is of magnetic material. In FIG. 1 ashaft 10 of magnetic material is rotated magnetic material is rotatedabout its axis so that a portion 12 of it is magnetised by theaxially-spaced north-south poles NS of a magnet arrangement 14. This maybe conveniently an electromagnetic which enables the magnetisation to bereadily controlled. The magnet system may be moved about the shaft. Theresult of this magnetisation is to produce an annular zone of surfacemagnetisation 16 as shown in FIG. 2a having NS poles as indicated. Itextends as an annulus about the shaft axis having the remanentmagnetisation of the same polarity around the axis of the shaft andaxially-directed. As indicated, the annular magnetisation tends to forma closed flux path within the shaft interior to annulus 16 so that atoroid of magnetic flux is established about the shaft axis. What isimportant is the magnetic field detectable exteriorly of the shaft aswill be shortly explained.

[0005] The toroidal flux concept can be enhanced as is shown in FIG. 2bwhich shows a surface adjacent annular magnetised zone 16 within whichan interior annular magnetised zone 18 of opposite polarity isestablished.

[0006] The two zones combine as shown to provide the torus of closedloop magnetic flux. The magnetisation is obtained by a two-stepprocedure. Firstly a deeper annular region of the polarity of zone 18 isformed by the magnet 14. Then the surface adjacent zone 16 is formed byreversing the magnetisation polarity of the surface adjacent region ofthe deeper region.

[0007] Turning to the practical utilisation of the resultant transducerelement, reference is made to FIG. 3a which shows the magnetic field ofzone 16 as seen at the surface of the shaft in the absence of torque.The arrow Mf indicates a fringing field which will extend generally inthe axial direction between the poles of region 16 in the ambientmedium, usually air.

[0008]FIG. 3b shows the effect of putting the shaft, and thus transducerelement portion 12, under torque in one direction about the axis A-A ofshaft 10. The longitudinal field in zone 16 is skewed as shown by thearrows (the skew is exaggerated for clarity of illustration). Theexternal fringing field is likewise skewed or deflected as representedby magnetic vector Mf FIG. 3c). Also generated is a vector component Mswhich in this embodiment extends circumferentially about thecircumference of shaft 110. The component Ms is tangential to the shaftat any point, that is perpendicular to the local radius. It is the Mscomponent that provides the component for measuring torque by means ofan appropriately oriented magnetic field sensor or group of sensors Msis a function of torque. If the torque is in the opposite direction thedirection of Ms is reversed. At zero torque, Ms has a zero value.

SUMMARY OF THE PRESENT INVENTION

[0009] The present invention is also based on a development of thelengthwise or axially-directed magnetisation of a portion of amagnetisable shaft. However, in the transducer element of the presentinvention the torque-dependent external field emanated by the transducerelement has axial and radial components which are axially-shifted as afunction of torque. This will be described subsequently and is asurprising result. More surprisingly in tests performed on a transducerelement produced by the magnetisation process of the present invention,it has been found that there is no detectable circumferential ortangential component Ms: or at least any such component is so weak as tobe lost in noise.

[0010] The creation of a transducer element of the invention having theabove field distribution characteristics is described below. Themagnetisation of the element is also accomplished by relative rotationabout an axis of a shaft in a magnet system generally as illustrated inFIG. 1. A greater depth of magnetisation is obtained by using a magnetsystem comprising permanent magnets forming a horseshoe magnet usingstronger magnets than were used previously. More particularly the magnetpoles adjacent the shaft have been relatively wide in the axialdirection as compared to their thickness in the circumferentialdirection.

[0011] The processing for creating the transducer element in a shaftfalls broadly into two operations with a third operation that mayfollow: a magnetic preparation which may be referred to for brevity asde-gaussing or magnetic cleansing; and thereafter a magnetisationprocedure. The magnetic preparation may be preceded by pre-treatment ofthe shaft which may include mechanical operations on it. The magneticpreparation is considered to be new and inventive in its own right andmay be applied generally to shafts or other objects requiring a magneticfield to be established in them for use as a transducer element. Themagnetic preparation (pre-magnetisation) procedure to be described canbe summarised as providing a magnetically cleansed part in which thedesired magnetic field is then established. The magnetisation operationmay be followed by a post magnetisation procedure somewhat similar tothe pre-magnetisation procedure.

[0012] Aspects and features of the present invention for whichprotection is presently sought are set forth in the claims followingthis description.

[0013] One aspect of the invention is a magnetic transducer element asset forth in Claim 1. Another aspect of the invention is a magnetictransducer element as set forth in Claim 8. A further aspect of theinvention is a transducer arrangement comprising a magnetic transducerelement and at least one magnetic field sensor as set forth in Claim 14.Yet other aspects of the invention lie in a method of preparing amagnetisable member for magnetisation as set forth in Claim 20 and amethod of magnetising a portion of a member as set forth in Claim 23.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 shows a magnetisation procedure for longitudinalmagnetisation of a portion of a shaft according to a prior proposal;

[0015]FIGS. 2a and 2 b illustrate the form of the magnetic fieldestablished in the shaft portion of FIG. 1 according to a single-step ortwo-step magnetisation procedure respectively;

[0016]FIGS. 3a and 3 b illustrate the longitudinal magnetisationaccording to FIGS. 1, 2a and 2 b in the quiescent and torque staterespectively, and FIG. 3c is a magnetic vector diagram pertaining to thetorqued state;

[0017]FIG. 4 shows a shaft having an integral portion thereof beingmagnetised in accordance with the present invention;

[0018]FIG. 5 is an end view of the shaft and magnet assembly of FIG. 1;

[0019] FIGS. 6 to 6 c illustrate steps in the magnetisation procedurefor the shaft using the magnet assembly shown in FIGS. 4 and 5;

[0020]FIG. 7 is a diagrammatic representation of the magnetisationprocedure;

[0021]FIG. 8a is an illustration of the magnetic flux established in thetransducer element in a shaft, FIGS. 8b and 8 c being sections on thelines A1 and A2 respectively, and FIG. 8d a further illustration of thetransducer field;

[0022]FIGS. 9a and 9 b show sensor orientations for obtaining axial,radial and tangential magnetic profiles all to be taken as a function ofaxial position;

[0023]FIG. 10 is an axial field magnetic profile;

[0024]FIG. 11 is a radial field magnetic profile;

[0025]FIG. 12 is a circumferential field magnetic profile;

[0026]FIG. 13 is an explanatory diagram of magnetic flux direction inthe vicinity of the transducer element;

[0027]FIGS. 14a to 14 c are explanatory diagrams relating to the effectof torque on the magnetic field distribution;

[0028]FIGS. 15a and 15 b are diagrammatic illustrations for purposes ofexplanation of magnetic fields established in a transducer region by amagnetic source, FIG. 15b relating to the practice of the invention asshown in FIGS. 4 and 7;

[0029]FIG. 16 is a circuit diagram of apparatus for use in apre-magnetisation and post magnetisation procedure for a shaft;

[0030]FIG. 18 is a generalised representation of the curves of FIG. 10;

[0031]FIG. 19 is a generalised representation of the curves of FIG. 11;

[0032]FIG. 20 shows sensor pairs for radial field measurement on thebasis of FIG. 19;

[0033]FIG. 21 shows the arrangement of one sensor pair;

[0034]FIG. 22 shows a circuit for obtaining a gain compensated torqueoutput signal;

[0035]FIG. 23 shows the placing of a pair of axially oriented sensors onthe basis of FIG. 18;

[0036]FIG. 24 shows a magnetisation system and process to provideguard/keep fields; and

[0037]FIG. 25 shows diagrammatically the resultant fields in a shaft.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0038] The invention will be particularly described with reference tothe magnetisation of a portion of a shaft to form a transducer element.The body or part in which the desired magnetisation is to be establishedmay be more generally referred to as the sensor host. The section whichnext follows relates to the magnetisation of a magnetically clean sensorhost. The magnetic preparation (pre-magnetisation) of the sensor host,specifically the shaft, is described subsequently as is thepost-magnetisation procedure. The resultant transducer element isdiscussed in the context of non-contact sensing of a rotating shaft.

[0039] The Magnetising Assembly

[0040]FIG. 4 shows a magnetically-cleansed shaft 20 of circularcross-section subjectable to torque applied about its axis A-A. Theshaft 20 may be mounted for rotation about axis A-A. The shaft has asolid cross-section. The direction of rotation is clockwise (cw) asreferred to the shaft as seen from its right-hand end in the figure.FIG. 5 is a view from the right of FIG. 4. Mounted closely adjacent thecircumference of a portion 22 of the shaft is a magnetic source 30 whichcomprises a magnet assembly with a pair of powerful magnets 32, 34arranged radially lengthwise (in their NS direction) with respect to theshaft surface with opposite polarity. Their remote poles are connectedby a flux concentrator 36 forming a low reluctance bridge between themagnets to assist in generating or concentrating the magnetic flux 38from the opposite pole pieces or ends 32 a, 34 a entering the adjacentzone of portion 22 in order to magnetise it. The magnet assembly 30 thusprovides a U-shape or horseshoe magnet acting in a radial plane. It isconstructed in separate parts in order to make use of very powerfulpermanent magnets 32 and 34. The field strength achieved between themagnet poles and the sensor surface is greater than 4 kGauss andpreferably in excess of 5 kGauss. This requires a minimal spacingbetween the magnets and the surface as will be discussed.

[0041] As can be seen from. FIGS. 4 and 5, the magnets 32, 34 arealigned in the axial direction and the shaft 20 is rotated about axisA-A with respect to the magnets to induce South and North poles 24 and26 in an annular zone about axis A-A of portion 22. The magnetisationextends around the circumference of the shaft inwardly from the surfaceof the shaft. The depth of the magnetisation achieved in portion 22isalso important as will be discussed below. Portion 22 provides atransducer element responsive to torque applied about the axis A-A ofshaft 20 In FIGS. 4 and 5 the shaft is shown as is rotating with respectto magnet assembly 30 but it will be understood that the desiredrelative rotation can be achieved by rotating the magnet assembly aboutthe shaft or a combination of the two.

[0042] In the magnet assembly shown and used for the magnetised shaft,tests on which are reported below, the axial width w of each pole piece32 a, 34 a (and of each magnet 32, 34) is substantially greater than thethickness t in the circumferential direction. Furthermore the gap gbetween the pole pieces 32 a, 34 a is also substantially less than thewidth w.

[0043] By way of example, the nature of the externally-detectablemagnetic field to be described more fully below was achieved bymagnetisation performed on a shaft of 18 mm. diameter consisting of ahigh performance FV520B steel. Each magnet 32, 34 had a width w of 15mm. and a thickness t of 4 mm. The gap g utilised was 2 mm. In themagnetisation process, the spacing d between the pole pieces 32 a, 34 aand the shaft surface was kept as far as possible below 2 mm. Increasingthe gap will reduce signal gain (slope) and signal linearity. Signalgain is further discussed below. The magnetisation procedure seeks tomagnetise the portion 22 to saturation to a depth such as indicated inFIG. 7 (annulus 42). Also diagrammatically shown in FIG. 4 is themagnetic flux 40 that is emanated exteriorly of the shaft portion 22between the North and South poles 24 and 26. It is this exterior fluxand its behaviour under torque which has yielded the surprising resultsto be described.

[0044] The Magnetising Procedure

[0045] More detail will now be given with reference to FIGS. 6a-6 c ofthe magnetising procedure or magnetic programming as it may also becalled. In practice the movements and their timing will be effected bymeans of an appropriately constructed machine. The magnetic source ormagnetic programming unit (MPU) will be assumed to be the singlehorseshoe assembly 30 already described. Again it is assumed that theshaft to be magnetised has been magnetically cleansed.

[0046] In FIGS. 6a-6 c, MPU 30 is shown in a sequence of steps 1), 2)and 3) in which it is moved along a radially-directed axis toward therotating shaft 20 from a neutral or non-active position (FIG. 6a) to afully magnetically engaged position (FIG. 6b) and then back to a neutralposition (FIG. 6c). The distance D₁ is the spacing between the MPU 30and the shaft surface in the neutral position while D₂ is the minimumspacing (d in FIG. 5) in the fully magnetically engaged position. Theshaft rotates at an angular velocity V_(1.) The linear speed with whichthe MPU 30 moves forward toward the shaft 10 is V₂, while the speed ofretraction from the shaft is V₃.

[0047] The environment in which the magnetisation procedure is performedshould be as free as possible from the generation of magnetic fieldsthat could influence the programming of shaft 20 with the desiredmagnetic field. In particular any means by which the MPU 30 is movedshould be designed to avoid setting up unwanted fields in the vicinityof the shaft. Magnetisation proceeds as follows:

[0048] Step 1) (FIG. 6a): While MPU (30) is located in its nonactiveposition at distance D1 from the shaft 10, the shaft commences rotationat an angular velocity V₁ which is maintained constant throughout theprocedure. D1 is sufficient that any magnetic flux directly orindirectly from the MPU will not have any permanent effect on the shafthost.

[0049] The rotational speed V₁ is not critical but as stated it isimportant to keep it constant through the entire magnetisationprocedure. In general V₁ may lie in the range 10-3000 rpm. One factor tobe taken into account in setting the value of V₁ is a parameter calledzero-torque offset. This is discussed further below.

[0050] Step 2) (FIG. 6b): Having established rotation of the sensor hostat V₁, the MPU 30 is then moved toward the shaft 10 at relatively slowspeed V₂. The value of V₂ is partially dependent on V₁. At higher valuesof V₁, V₂ can be increased. Typically at V₁=2800 rpm, V₂ can be 1 to 2mm/sec. It is generally desirable to move the MPU as close to the shaftsurface as possible consistent with avoiding the permanent magnetscontacting the surface, i.e. the fully magnetically engaged position—seealso FIG. 7. Contact may modulate the rotational speed of the shaft orthe movement of a control mechanism guiding the MPU. The distance D₂between the MPU and the surface of the sensor host should not only bemade as small as possible but should be maintained constant to a highdegree of accuracy. To this end a feedback control means for sensing theposition of the MPU 30 with respect to the surface of the sensor hostcan be employed so as to achieve the desired control.

[0051] The MPU is maintained in the fully magnetically engaged positionfor a number of rotations to achieve saturation magnetisation of theportion 22 of the shaft. It is presently preferred that the depth ofmagnetisation be more than 30% of the radius of the shaft but goingbeyond a depth of 60% may reduce the sensor performance.

[0052] Step 3) (FIG. 6c); While still maintaining shaft rotation at V₁,the MPU is retracted or withdrawn from the fully magnetically engagedposition to the neutral position at a speed V₃ directly related to V₁.Once back in the neutral position the rotation of the shaft is stoppedand the shaft can be removed. The shaft should now have a well-definedmagnetisation of the portion 22 as indicated in FIG. 4. The nature ofthis magnetisation is further described with reference to FIGS. 7, and 8a to 8 c. V₃ would normally be substantially less than the forwardadvance speed V₂. For a value of 2800 rpm, a value of V₂ of 1-2 mm/sechas been mentioned: a value of V₃ of 0-5 mm/sec or less would beappropriate for retraction, preferably 0.25 mm/sec or even less. Theretraction should not engender any disturbance of the desiredmagnetisation established in Step 2).

[0053] Step 4) (optional): The magnetised shaft may be subjected to apost-magnetisation stage which is similar to but carried out at a lowerlevel than the pre-magnetisation cleansing description below.

[0054] Before moving on to FIGS. 7 and 8a-8 c, the concept of zero-forceoffset will be explained. This was mentioned in Step 1 with reference tothe choice of rotational velocity V₁.

[0055] Start with the situation in which the shaft is magnetised asdescribed above under zero or near-zero torque in the shaft. As willbecome more apparent from the response graphs discussed below, when themagnetised portion is used as a torque transducer element, it provides amagnetic field output which a) is a function of torque, b) has apolarity that is dependent on the direction of torque, clockwise (cw) orcounter-clockwise (ccw), and c) has an essentially zero value at zerotorque. However, if the magnetisation procedure is performed while theshaft is under torque, that will be the torque at which the outputsignal passes through zero. On relaxing the shaft to zero torque anon-zero quiescent output is obtained. The polarity of the quiescentoutput depends on the direction of the torque applied in magnetisation.The phenomenon can be put to practical effect in a technique known aspre-torquing disclosed in PCT application PCT/GB00/01103 published underthe number WO00/57150 on 28 Sep. 2000.

[0056] The magnetisation procedure described above may be applied toform multiple transducer elements along the shaft sequentially orsimultaneously with an appropriate number of MPUs). The elements may begiven different polarities and, if required, be pre-torqued. Presentpractice indicates a preference for magnetising different axial portionsof the shaft pertaining to different transducer elements simultaneously.This lessens the possibility of the strong field used for magnetising asubsequent portion affecting a previously magnetised portion. Multiplemagnetised portions can also be employed where a transducer elementportion is flanked by guard or keeper portions each of opposite polarityof magnetisation to the transducer portion (FIG. 24).

[0057]FIG. 7 is a side view of the rotating shaft showing themagnetisation of portion 22 during step 2) of FIG. 6b. the MPU 30 is inthe fully engaged position and the generated flux 38 magnetises anannular region 42. The magnetisation in this region is axially-directed.

[0058] The nature of the deep axially-directed magnetisation achievedwith the above procedure will be more fully described with reference toFIGS. 8a-8 d.

[0059]FIG. 8a shows an axial cross-section of a shaft 20 having aportion 22 magnetised by the steps described above to provide theannular region 42 (shown shaded) having North and South poles 26 and 24.The poles are, of course, not as clearly delimited as the drawing showsfor clarity of illustration.

[0060]FIGS. 8b and 8 c are radial cross-sections taken on A1 and A2respectively in FIG. 8a The cross-sections are drawn to indicate theflux polarity as seen looking towards portion 22 from the outside. ThusFIG. 8b shows the internal flux of region 42 directed (S) toward theNorth pole of the region while FIG. 8c shows the internal flux directed(N) away from the South pole of region 42. The annular magnetised regionforms a closed magnetic loop 44 interiorly of the region and a lesserexternal loop 40 is established outside the shaft. Both loops will be oftoroidal form about shaft axis A-A.

[0061] Experiments have shown that the inner surface of the loop 44becomes of essentially zero or near zero radius. This is indicated bythe polarities shown at the centre of each of FIGS. 8b and 8 c and isfurther diagrammatically illustrated by the flux distribution of FIG.8d. This enhanced magnetisation flux distribution through the axial coreof shaft 20 is a function of the radial depth of the region 42. Asalready mentioned, this region Us magnetised to saturation. Theexperimental results reported below with reference to FIGS. 10, 11 and12 are for a shaft 10 magnetised in accord with the steps abovedescribed to have the kind of flux distribution just described. It willbe appreciated by those skilled in the art that the determination offlux distribution within solid magnetised objects is not easy. In theinvestigations we have performed probes have been inserted in fineaxially-directed bores within the shaft.

[0062] For practical torque measurement purposes, the field distributionused is that of the external field 40. The practical consequences ofproducing a transducer element by means of the magnetisation proceduredescribed above are surprising and unexpected.

[0063] External Magnetic Field Profiles

[0064] There will now be described the measurement of magnetic fieldprofiles made with a shaft having a portion magnetised in accord withthe magnetisation procedure described and bounded axially bynon-magnetic portions, which like the magnetised portion were previouslymagnetically cleansed in the manner to be described. That is the wholeshaft was cleansed in the manner to be described.

[0065]FIGS. 9a and 9 b illustrate the three orientations of sensors B bywhich external field distribution is investigated. FIG. 9a shows a shaft20 having a magnetised portion 22 as above described the centre of whichin the axial direction is taken to be at radial plane 28. Threedifferent magnetic profiles were measured using directional sensorsoriented in the manner shown in FIGS. 9a and 9 b. The sensors used wereof the saturating inductor type such as described in PCT publicationWO98/52063 in which the field sensitive inductor is typically a fewmillimetres in length. Other sensor devices commonly used for measuringmagnetic fields are Hall effect sensors, the field sensitive element ofwhich is very much smaller, and directional magnetoresistive sensors.All these sensors have a figure-of-eight response pattern with a broadangle of near maximum response and sharper nulls perpendicular to theaxis of maximum response.

[0066] Three sets of measurements were performed using three differentsensor orientations with respect to the shaft 10 as represented byinductors 50 x, 50 y, 50 z in FIGS. 9a and 9 b. All measurements weremade with the relevant sensor adjacent to but not in contact with theshaft surface. The shaft was rotated about its axis with a constantapplied torque. Three different torque values were used. Themeasurements were taken one orientation at a time.

[0067] Sensor 50 x is oriented in the axial direction (x-axis) alsoreferred to as the in-line direction. Measurements of the magnetic fieldBx parallel to the axis (the in-line field) as a function of x weremade. The sensor was moved in the axial (x)-direction to obtain thefield profile of FIG. 10.

[0068] Sensor 50 y is oriented radially of the shaft axis A-A to providea measurement of radial field B_(y). It is also moved in the axial (x)direction to provide a profile of the radial field B_(y) as a functionof x at a constant radius. This profile is seen in FIG. 11.

[0069] Sensor 50 z is oriented tangentially of the shaft to respond tothe circumferential or tangential field B_(z). It is also moved in theaxial (x) direction to provide a profile of the tangential field B_(y)as a function of x at a constant radial distance from the shaft. Thisprofile is seen in FIG. 12.

[0070] The axial field, radial field and tangential field profiles givenin the curves of FIGS. 9a-9 c respectively will now be furtherdiscussed. Each profile was obtained by means of the same sensororiented in the appropriate 50 x, 50 y, 50 z position.

[0071]FIG. 10 shows three profiles of a shaft magnetised by theprocedure described above with a sensor 50 _(x) mounted at 2.5 mm. fromthe shaft surface. The shaft is run at 2800 rpm and profiles were takenat torque values of 0 (zero), +80 and −80 Nm. The abscissa axis of thegraph is the x distance from the axial centre 28 of the transducer. Theordinate axis is an output signal value in mV representing the sensedfield. The output shows an offset. The ordinate zero is at about 2300mV. The profile obtained is of similar shape for all three torques butit is seen that the profile shifts axially with torque. The zero torquecurve peaks at the centre line 28. The peaks for the +80 and −80 Nmcurves have the same peak magnitude as the zero torque curve but areshifted to opposite sides of the centre. Thus a torque causes an axialshift in the axial field B_(x) the magnitude of which shift is afunction of torque and the polarity of which is dependent on thepolarity of the applied torque.

[0072]FIG. 10 also exhibits some other characteristics of interest. Themeasured B_(x) field enters a region of opposite polarity adjacent thepoles of region 22 before dropping towards zero as the sensor is movedaway from the poles. The change in polarity is explained below withreference to FIGS. 13 and 14. The poles are at about ±15 mm. from thetransducer centre line 28. The measured field polarity is in the samesense at the two pole regions but exhibits a torque-dependentdifferential in amplitude. It will be seen that as compared with thezero torque curve, which is near symmetrical about the x=0 point, theoutput measured in the region adjacent one pole (at about 15 mm.distance) is enhanced for one polarity of torque and decreased for theother polarity of torque but the enhancements/decreases are of oppositesense in the pole regions. It may be surmised that the axial field isthe being tilted from one pole towards the other with the direction oftilt dependent on torque.

[0073] Another characteristic, which appears also in FIGS. 11 and 12, isthat there appear to be points P adjacent but just axially outside thepoles through which the curves all pass. These are referred to herein aspivotal points.

[0074]FIG. 11 is a similarly produced magnetic field profile, in thiscase for the radial field B_(y). The sensor 50 _(y) is mounted at 2.5mm. from the shaft surface.

[0075] In FIG. 11 the output values in mV are also shown with an offset.The ordinate zero is at 2500 mV. Looking at the 0 (zero) Nm curve, itreaches the same peak magnitude at each pole region but the signalpolarities are opposite. Between the two regions is a smooth variationpassing through zero at X=0 (allowing for experimental error). The ±80Nm curves show the same characteristics as was exhibited in the axialfield measurement. They are x-shifted with respect to the zero torquecurve. At one pole region the peak of one has increased and the otherdecreased relative to the zero torque curve and the sense of theincrease/decrease is opposite at the two pole regions. This againappears to be consistent with a tilting of the magnetic field patterndependent on the polarity of the applied torque. The curves all cometogether at pivotal points or small regions P just beyond the poles.

[0076] Finally FIG. 12 illustrates that the magnetisation produced inthe shaft has essentially no circumferential or tangential componentB_(z) at any value of X.

[0077] Without attempting to theorize about the nature of thetorque-dependent magnetic field emanated by the transducer element 22(the “tilt” concept is put forward as a possible effect on the field),it can be seen that both the axial field and radial field provide thepotential for a magnetic-based transducer system for measuring torque.In particular the parallel straight line regions each side of theabscissa zero in FIG. 10 may be utilised for torque measurement. It hasbeen found that within elastic limits a linear relationship betweentorque and magnetic output exits. Similarly the parallel straight lineregion extending about the abscissa zero in FIG. 11 may be used. It alsoprovides a linear relationship between torque and magnetic output.

[0078] In considering the results demonstrated by the curves of FIGS. 10and 11, reference is made firstly to FIG. 13. This illustrates themagnetic flux direction represented by vector arrows measured adjacentthe is surface of the sensor host (2 mm. from the shaft surface). Eachspot or dot indicates an axial measurement point and each vector arrowthrough the spot indicates the direction of the flux at that point,though not its relative magnitude. The measurements extend over and justbeyond the magnetised portion of the shaft. The poles lie in the regionsmarked N and S. The shaft has zero applied torque (or any other force).

[0079] The field is essentially axially directed at the centre line 28,becomes increasingly radial moving towards the poles, the radialcomponent peaking adjacent the pole regions with zero axial component.Moving further beyond the poles the radial component decreases and theaxial component increases but now in the opposite sense to that at thecentre line. It must be borne in mind that FIG. 13 is concerned onlywith direction not with magnitude. It is considered that the vectordistribution shown in FIG. 13 is consistent with the measured magnitudedistributions for zero torque of FIGS. 10 and 11.

[0080]FIGS. 14a to 14 c show a simplified magnetic model relating to the“tilt” concept already mentioned in regard to the change in the axialand radial field distributions under torque. FIGS. 14a-c show fieldsrelating to torque applied in one sense (say cw), zero torque and torqueapplied in the other sense (ccw) respectively.

[0081] Looking first at FIG. 14b, there are shown fans 60 and 62 ofmagnetic flux lines taken as emanating from points 64 and 66 adjacentthe poles of magnetised region 22 of the shaft. The directions of theflux lines are represented by the arrows. The magnitude of the field isnot indicated. The angle of the fans is expressed with respect to adatum 68 and 70 respectively. In FIG. 14b each datum is aligned with thecentral arrow 61, 63 of the respective fan. Under cw torque in FIG. 14athe fans “roll” about points 64 and 66. Under no torque the two fansaligned with the datums are at the same angle (though pointing inopposite directions) to the surface of the shaft, i.e. αp=αn. Undertorque they roll in the same direction so that the projected lines ofthe arrows 61, 63 shifts with respect to the datums 68, 70. If theangles are taken with reference to the surface of the shaft 20, theangle of αp of fan 60 decreases while the angle αn of fan 62 increasesfor cw torque in FIG. 14a. The converse occurs when the torque isapplied in the ccw direction as illustrated in FIG. 14c.

[0082] Details of a particular magnet configuration acting as a magneticsource have been given above with reference to FIGS. 4 and 5. Furtherexplanation of the creation of the magnetised region within the shaftwas given with reference to FIG. 7 and FIGS. 8a-8 c. The magnetic source30 in FIG. 7 is of a similar configuration to that seen in FIG. 4 tocreate a transducer element which provides an external magnetic fieldhaving the profile characteristics described above. Some further aspectsof the parameters of the desired magnetic pole configuration will now bediscussed with reference to FIGS. 15a and 15 b which contrast aconfiguration in FIG. 15a, which as best presently understood does notlead to a degree of magnetisation and a field sufficient for the presentinvention, with a configuration in FIG. 15b which does produce thedesired external field. Referring to FIG. 15a it shows a magnetic source30 comprising magnets 32′ and 34′ oppositely poled with respect to theregion 22′ of a shaft 20 that is to be magnetised to provide atransducer element. The magnets 32′ and 34′ are linked by a fluxconcentrator 36′ to provide a U-shape magnet acting on the shaft 20.What is different about the assembly of FIG. 15a is the relatively widegap g between the poles and particularly the pole ends 32′a and 34′a.Each magnet 32 and 34 tends to form a magnetic field 35 about itself theflux in which does not contribute to the flux available to enter andmagnetise shaft portion 22′ to remanance magnetisation. Furthermore theflux which does find a path through the portion 22 tends to beconcentrated near the surface in a surface-adjacent zone 43. As themagnets 32′ and 34′ are brought nearer together (g is reduced) the flux35 linked about each individual magnet reduces and more flux isconcentrated in the portion 22′ as seen in FIG. 15b and to a greaterdepth. This is the situation seen in FIG. 4 and FIGS. 7 and 8a-8 d inwhich in FIGS. 4 and 7 the arrowed loops indicate the flux circulatingthrough the magnet assembly and the adjacent portion 22′ of the shaft20. In addition the depth of magnetisation within the portion 22′ isenhanced by having relatively wide (w) pole ends 32′a and 34′a. A ratioof w/g of about 7 or greater appears satisfactory.

[0083] Electromagnet Alternative

[0084] The permanent magnet assemblies so far discussed have theadvantage that very high magnetic field strengths can be achieved withreasonably small dimensions. They do, however, have the disadvantagethat their field strength cannot be easily altered nor can they beswitched off. As an alternative an electromagnet assembly energised withdirect current (D.C.) can be used as a magnetic source for the magneticprogramming of the sensor host. Because of the control possible withelectromagnets it becomes possible or more readily possible to match thefield strength between magnets to achieve the desired sensorperformance: to adjust the effective field strength between the magnetassembly and sensor host to improve significantly the rotationaluniformity of the measurement signal: to adjust the desired full scalemeasurement range: to quicken the programming process, particularly inthe withdrawal or retraction phase of Step 3) above. The electromagnetdoes not necessarily have to be withdrawn from the sensor host in Step3) or advanced toward it in Step 1), but the procedure of steps 1) to 3)can be emulated by controlling the electromagnet current. Step 1) can beimplemented by ramping up or increasing the energising current while theeffect of the MPU retraction is obtainable by reducing or ramping downthe energising current for the electromagnet. A combination of movementand current control may be employed.

[0085] The maximum flux density achievable in electromagnets is lessthan that in permanent magnets so that for a given usable field strengthin the sensor host an electromagnet system will be physically largerthan a permanent magnet one.

[0086] Pre-Magnetisation Procedure

[0087] The pre-magnetisation process for the sensor host will now bedescribed, specifically in relation to demagnetising or de-gaussing ashaft. This procedure is important to obtaining the field distributioncharacteristics described above. The shaft as received may have beensubject to various mechanical and/or heat treatment operations whichdifferentially affect the magnetic domains within the material. It mayhave been subject to and have acquired undefined magnetic fields. Suchunknowns will be deleterious to transducer performance. Thus in mostcases the shaft is to be put through a pre-magnetisation procedure toput it into a magnetically-defined state which has been referred toabove as magnetically cleansed.

[0088] The degree of demagnetisation required is partially dependent onthe magnetisation to be applied thereafter. For example if the magneticprogramming to create the transducer element uses a relatively low levelof magnetic field strength, the more important it becomes to completelydemagnetise the sensor host. In this context magnetic cleansing meansthat the de-gaussing or demagnetisation procedure results in that themagnetic direction of the individual grains of the shaft material israndom so that no grouping of magnetic domains in any particulardirection exists. The existence of magnetic domain grouping to providesome organised magnetic orientation of individual grains leads todeficiencies in the magnetised transducer element. For example,increased offsets of the measured magnetic signal; non-uniformity of thesignal as a function of the rotational angle of the shaft; and lowerstability over time of the transducer element.

[0089] The magnetic cleansing should extend well beyond the region atwhich the magnetised transducer element is to be formed, e.g. preferablythe whole shaft should be demagnetised so that there are no undefinedlocal magnet systems in the sensor host. In particular “bar-magnet”formations parallel to the shaft axis may travel over time within theshaft to affect the sensor specification on any ongoing basis.

[0090]FIG. 16 illustrates an apparatus for magnetic cleansing. Itcomprises a demagnetising coil 80 would in hollow solenoid fashion, amains powered transformer arrangement 82 and a current limiter 84. Foran 18 mm. diameter shaft a suitable coil was about 300 turns off about30 cm. diameter of a heavy current carrying capacity cable. The outerconductor of a heavy coaxial cable coiled into a solenoid coil proved tobe suitable. The transformer arrangement 82 comprises a variabletransformer 86 connected to a 110 or 240 VAC mains AC supply. This is inturn connected to an isolation transformer 88 capable of safelydelivering 10 amperes or more at its secondary at voltages at up to say48V. The coil 80 is connected to the secondary of transformer 88 throughthe current limiter 84 which may be a resistor, e.g. a power rheostat,or more elaborate electronic device. The current limiter may be omittedprovided steps are taken to monitor the current through the coil. Atypical coil resistance would be about 100 milliohms. The variability ofthe transformer arrangement enables the current to be controlled asdesired.

[0091] The coil 80 is energised and the full length of the shaft ispassed through the coil while the coil is energised at 8-10A Thisproduces a de-gaussing field of is about 1 kGauss. Typically one islooking to achieve fields in the 500-1200 Gauss range. The shaft may bemounted on a movable jig to move it along the axis of the coil and themovement continues as the far end of the shaft leaves the coil so thatthe field to which the shaft is subjected gradually decreases. There maybe other ways of achieving the de-gaussing procedure including controlof the coil current as a function of the axial position of the shaftwith respect to the coil.

[0092] This pre-magnetisation is considered to have more generalapplicability to a wide range of sensor host shapes (shafts, discs etc.)and to a wide range of magnetic transducer types, includingcircumferentially-magnetised.

[0093] Post-Magnetisation Procedure

[0094] The optional Step 4) of a post-magnetisation step following themagnetisation procedure described above is performed in the same manneras the pre-magnetisation procedure but at a lower level of magneticfield. This step may also be applied more generally to stabilise sensorhosts magnetised in other ways such as set out above.

[0095] In the post-magnetisation procedure of Step 4), the magnetisedshaft is again passed axially through the energised solenoid coil 80.However, the AC current through the coil is of an order of a magnitudelower than for the pre-magnetisation procedure. In the pre-magnetisationexample given above, the 8-10A current employed for pre-magnetisation isreduced to say 0.5-1A for post magnetisation. The current is at a valuewhich does not change the basic magnetic pattern sought to beestablished but, as best can be surmised, it reduces or knocks-backparasitic fields that may be present after the magnetisationproceedings. It has been found that the post-magnetisation step improvesthe uniformity of the output signal with rotation of the shaft, offsetsover time and the final sensor stability generally.

[0096] Reverting to the magnetisation procedure and particularly to Step2) of it, it has already been mentioned that the distance D₂ (FIG. 6b)should be kept as small as possible. Actual contact with the shaftsurface should be avoided. Furthermore, the distortions should be keptconstant as small variations can greatly influence the flux which entersthe sensor host. For example, a position control feedback system basedon a laser distance sensor acting between the MPU 30 and the shaftsurface can be employed. Such a system will be usable on shafts ofnon-circular cross-section. The field between the MPU poles and thesurface of the sensor host is very large typically in the region of the±1 kGauss to ±6 kGauss.

[0097] During the magnetisation procedure, particularly Step 2), themagnetisation achieved can be measured, for example at a point remotefrom the MPU 30, e.g. the is opposite side of the shaft. an independentsensor device is set up at this point to measure an external field thatcan be correlated with the internal stored field. The magnetisationprocedure can continue until some wanted sensed field is obtained.Programming by means of an electromagnet system is advantageous here inthe greater control that can be exercised.

[0098] The magnetisation level in the sensor can be monitored in realtime and the electromagnet current adjusted accordingly. An alternativeis a stepwise approach in which the electromagnet current is set to agiven magnetisation level; the electromagnet is switch off while amonitoring measurement is made; and the electromagnet is re-energisedfor a lower or higher level of magnetisation dependent on themeasurement. The monitoring measurement and re-magnetisation steps canbe repeated until the desired result is achieved.

[0099] The real time and stepwise procedures apply in the theory to theuse of permanent magnets by adjustment of position but the degree ofcontrol required is more difficult to realise.

[0100] Torque Measurement Systems

[0101] The following description is concerned with the implementation oftorque measurement systems based on the curves of FIGS. 10 and 11, andparticularly the straight line regions of them previously noted.

[0102]FIG. 18 is a generalised representation of the axial profilecurves of FIG. 10 (the polarity is reversed) in which 90 indicates thezero torque curve and 92 and 94 are relevant segments of the torqueshifted curves resulting from opposite directions of torque. A sensor orarray of sensors is placed in non-contacting position closely adjacentthe shaft at axial positions such as X₁+ and X₁− in the regions wherethe curve segments are parallel and most nearly linear. The sensors areoriented axially. Linearity of field strength with axial position at agiven torque is not essential but is desirable and aids in calibrationand calculation of torque values. It will be seen that a sensor placedat X₁+ or X₁− will produce a signal representing B_(x) that is afunction of torque. The output at X₁+ is of opposite sign to that atX₁−. A sensor can be placed at each of the two positions and the signalscombined to add together. This is further described below with referenceto FIG. 23.

[0103] The radial case is illustrated in FIG. 19 which is the centralsection of the curves of FIG. 11 (the polarity is reversed). 96indicates the zero torque curve going through the origin region, and 97and 98 relevant torque shifted segments. This is exploited as shown inFIG. 20. Single sensors or preferably opposed pairs 52′, 54′ of sensorsare placed to each side of the centre line of the transducer element(the magnetised portion 22 of shaft 20) at positions such as X₁ and X₂,preferably of equal X magnitude from the centre plane 28. FIG. 21 showsthe advantage of an opposed pair of sensors (e.g. 52′) which areradially oriented and diametrically opposite. The sensor pair areconnected to additively combine their opposite polarity outputs A₁ andA₂ to provide a combined signal A=A₁+A₂. However, common mode effectssuch as the Earth's magnetic field are cancelled. The outputs from theother pair of sensors are treated in the same way to obtain a combinedoutput signal B=B₁+B₂.

[0104]FIG. 19 shows that the value A−B remains constant at all torquelevels and represents the B_(y) v. X slope of the curves and thus can beexpressed as a “gain” factor for the transducer element. A or B or (A+B)is a torque dependent output function.

[0105]FIG. 22 is a circuit diagram illustrating how these signals can beused to provide a torque output signal which is compensated for changesin the gain factor. In FIG. 21 the sensor signals A and B from thetransducer element portion 22 are appropriately derived as by using thecircuitry 102 a, 102 b, as described in WO98/52063. The two signals areapplied to sum and difference units 104 and 106 respectively. The sumsignal (A+B)/2 is then applied as an input to a gain-controllableamplifier 108 to which the different unit output (A−B) is applied as again control signal. The output T of the amplifier 108 is a torquesignal T compensated for changes in the gain factor.

[0106] Reverting to the axial field distribution of FIGS. 10 and 18,FIG. 23 shows a pair of axially oriented sensors 56 a, 56 b locatedadjacent the magnetised transducer region of a shaft at positions suchas X₁− and X₁+ in FIG. 18. The two sensors can be connected to measuretorque (A+B)=A−(−B) and gain or slope A−B=A+(−B).

[0107] Guard/Keeper Field Regions

[0108] Mention has been made above of the provision of guard or keeperfields for the transducer element region. FIGS. 24 and 25 illustrate howthis may be done. FIG. 24 generally follows FIG. 4 but the magnet system130 of FIG. 24 is extended by two further poles. There are four radialmagnets of alternating polarity along and adjacent a shaft 120 andhaving a common extended flux concentrator 136. The magnetisationprocedure follows the steps of FIGS. 6a-6 c and is preceded andsucceeded respectively by the pre-magnetisation and post magnetisationoperations already described.

[0109] The magnets 132 and 134 act together to provide the magnetisedregion 122 for use as the transducer element. This has the external flux140. One outer magnet 133 coacts with magnet 132 to provide a magnetisedregion sharing one magnet (S) of portion 122 and a further oppositepolarity (N) closely adjacent to it axially. These poles are linked byflux 140′ which is not used in measurement. The other outer magnet 135similarly coacts with magnet 134 to provide a magnetised region sharingone pole (N) of portion 122 and a further opposite polarity pole (S)closely adjacent to it axially. The resultant closed loop magnetic fluxpatterns are shown in fine dash line in FIG. 25. The two outer keeper orguard regions 137, 139 act to prevent leaching of the active transducerregion 122 and prevent unwanted fields invading region 122 along theshaft, and to generally assist in stabilising the properties of thedesired transducer region. Guard regions for preventing unwanted fieldsinvading region 122 need not be longitudinally magnetised. They may haveother forms of defined magnetisation such as that known ascircumferential or circular magnetisation as described in the PCTapplication published under the number WO99/56099.

[0110] The foregoing description has been in terms of magnetising aportion of a shaft for torque measurement. The magnetisation describedmay be also applied to a transducer element in an elongate member—forwhich the term “shaft” may be generically used—subject to flexure, thatis to bending forces. In this case a transducer arrangement of the kinddescribed is used to measure applied force.

1. A magnetic transducer element comprising: a member of magnetisablematerial having an axis about which it is subjectable to torque or iscapable of flexing; said member being remanently magnetised to have anannulus of axially-directed magnetisation about said axis, said annulusof magnetisation emanating an external magnetic field that exhibits a)no significant component of circumferentially (tangentially)-directedmagnetic flux (with respect to said axis) externally of said member whenthe member is subject to torque applied thereto about said axis and/orto a force causing the member to flex about said axis, and b) asignificant component which is other than circumferentially(tangentially)-directed and which varies as a function of torque or offlexing of the member about said axis.
 2. A magnetic transducer elementas claimed in claim 1 in which said annulus of axially-directedmagnetisation extends inwardly from an external surface of said member.3. A magnetic transducer element as claimed in claim 1 or 2 in whichsaid member is an integral portion of a shaft (which term includeselongate members in general in respect of flexing) subject to torque orto flexing and in which said member is free of resident magnetic fieldsother than said annulus of magnetisation and said shaft is free ofparasitic fields in the vicinity of said member.
 4. A magnetictransducer element as claimed in claim 1, 2 or 3 in which said annulusof magnetisation forms a closed loop of magnetic field extending in aflux path which is interior to said annulus.
 5. A magnetic transducerelement as claimed in claim 2 or claim 3 or 4 when dependent on claim 2in which the radial depth of the annulus of saturated magnetisation isbetween 30 and 60% of the radius between the axis and said surface.
 6. Amagnetic transducer element as claimed in claim 1, 2 or 5 in which theexternal field emanated by said member has a distribution in thedirection of said axis, the axially-directed component of whichdistribution shifts in the axial direction as a function of torque overat least a portion of the emanated field distribution.
 7. A magnetictorque transducer element as claimed in claim 1, 2 or 5 in which theexternal field emanated by said member has a distribution in thedirection of said axis, the radial component of which distributionshifts in the axial direction as a function of torque over at least aportion of the emanated field distribution.
 8. A magnetic transducerelement comprising: a member of magnetisable material having an axisabout which it is subjectable to torque or is capable of flexing: saidmember being magnetised to have an annulus of axially-directedmagnetisation about said axis having poles of opposite polarity, saidannulus of magnetisation providing an axially-directed magnetic fieldexternally of said member, said magnetic field having a distribution inthe direction of said axis, at least a component of which shifts in theaxial direction when the member is subject to torque applied theretoabout said axis.
 9. A magnetic transducer element as claimed in claim 8in which said annulus of axially-directed magnetisation extends inwardlyfrom an external surface of said member.
 10. A magnetic transducerelement as claimed in claim 8 or 9 in which said component is anaxially-directed component of said distribution.
 11. A magnetictransducer element as claimed in claim 8 or 9 in which said component isa radial component of said distribution.
 12. A magnetic transducerelement as claimed in claim 8 or 9 in which an axially-directedcomponent and a radially-directed component shift of said distributionin the axial direction when the member is subject to torque.
 13. Amagnetic transducer element as claimed in any one of claims 6 to 12wherein the distribution of said field has pivotal points or regions ator axially beyond the poles of said member through which the axial orradial components, as the case may be, pass irrespective of thetorque-dependent axial shift.
 14. A transducer arrangement comprising amagnetic transducer element as claimed in any one of claims 1 to 4 andat least one magnetic field sensor located adjacent said annulus ofmagnetisation to sense the field emanated by said element.
 15. Atransducer arrangement as claimed in claim 14 in which said at least onemagnetic field sensor has a direction of minimum response in thecircumferential or tangential direction.
 16. A transducer arrangement asclaimed in claim 15 in which said at least one magnetic field sensor hasan axis of maximum response which lies in an axial direction to detectan axially-directed component or a radial direction to detect aradially-directed component of the emanated field.
 17. A transducerarrangement as claimed in claim 16 in which the axis of maximum responseis in a radial direction and said at least one magnetic field sensor islocated in alignment with or in the vicinity of the plane normal to saidaxis of said annulus of magnetisation that is axially centred withrespect to said annulus.
 18. A transducer arrangement as claimed inclaim 16 in which the axis of maximum response is in an axial directionand said at least one magnetic field sensor is axially located to oneside of the plane normal to said axis of said annulus of magnetisationthat is axially centred with respect to said annulus.
 19. A transducerarrangement as claimed in claim 18 wherein there are at least twomagnetic field sensors the axis of maximum response of each of which isin an axial direction and at least one magnetic field sensor is locatedto one side of said plane.
 20. A method of preparing a magnetisablemember for magnetisation comprising the steps of energising a coilhaving an aperture therethrough with A.C. and passing said memberthrough said aperture.
 21. A method as claimed in claim 20 in which saidcoil is energised to generate 2000 Ampere-turns or more.
 22. A method asclaimed in claim 20 or 21 in which said coil is wound in the form of asolenoid, said member is a shaft and said member is moved axiallythrough said solenoid.
 23. A method of magnetising a portion of a membercomprising preparing the member for magnetisation in accordance withclaim 20, 21 or 22, magnetising a portion of said member to establish anannulus of remanent magnetisation about an axis of said portion.
 24. Amethod as claimed in claim 23 in which the magnetising of said portioncomprises relative rotation of said member and a magnet system aboutsaid axis.
 25. A method as claimed in claim 24 in which the magnetsystem is a permanent magnet assembly and the magnetising includespositioning said permanent magnet assembly remotely with respect to saidmember where the permanent magnet assembly is non-active for magnetisingsaid member, relatively moving said permanent magnet assembly and saidmember to bring said permanent magnet assembly into magnetic engagementwith said portion of said member while said relative rotation iseffected in order to magnetise said portion, and relatively moving saidpermanent magnet assembly and said member back to the non-activeposition.
 26. A method as claimed in claim 25 in which the relativerotation commences while the permanent magnet assembly is in thenon-active position, continues while the permanent magnet assembly is inmagnetic engagement, and continues until the permanent magnet assemblyis back in the non-active position.
 27. A method as claimed in claim 24in which the magnet system is an electromagnet assembly and themagnetising comprises a combination of any of positioning theelectromagnet assembly, moving the electromagnet assembly relative tothe member, and controlling the energising current of the electromagnetassembly so as to emulate the relative positioning and moving steps ofthe permanent magnet assembly in claim 25 or
 26. 28. A method as claimedin claim 27 in which the energising of the electromagnet assemblycomprises a current increasing step, a constant current step toestablish said annulus of magnetisation and a current reducing step. 29.A method as claimed in any one of claims 23 to 28 further comprising apost-magnetisation procedure which comprises repeating the steps ofclaim 20 or claim 22 and 20 in which the energising current in the coilfor post-magnetisation is substantially less than the energising currentfor preparing the member for magnetisation, the annulus of magnetisationbeing retained.
 30. A method as claimed in claim 29 in which theenergising current for post-magnetisation is of the order of an order ofmagnitude less than the energising current for preparing the member formagnetisation.